1. Field of the Invention
The present invention is directed in general to magnetic resonance tomography (MRT) as employed in medicine for the examination of patients. In particular, the present invention is directed to a magnetic resonance tomography apparatus of the type wherein the homogeneity of the magnetic basic field is stabilized or by components referred to as shim iron plates.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been utilized as an imaging method for more than fifteen years in medicine and biophysics. In this examination method, the subject is exposed to a strong, constant magnetic field. As a result, the nuclear spins of the atoms in the subject are aligned, these having been previously irregularly oriented. Radio-frequency waves can then excite these “ordered” nuclear spins to perform a specific oscillation. This oscillation generates the actual measured signal in MRT that is registered with suitable reception coils. By utilizing location-dependent magnetic fields generated by gradient coils, the signals from the examination subject can be spatially encoded in all three spatial directions. The method allows a free selection of the slice to be imaged, as a result of which tomograms of the human body can be registered in all directions. MRT as a tomographic method in medical diagnostics is mainly distinguished as a “non-invasive” examination method on with a versatile contrast capability. Due to the excellent portrayal of soft tissue, MRT has developed into a method that is often superior to x-ray computed tomography (CT). MRT is currently based on the application of spin echo and gradient echo sequences that, given measuring times on the order of magnitude of a few seconds, achieve an excellent image quality.
The on-going technical improvement of the components of MRT devices and the introduction of fast imaging sequences open more fields of application in medicine to MRT. Real-time imaging for supporting minimally invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are only a very few examples.
The basic structure of one of the central parts of such a MRT apparatus as shown in FIG. 6. It has a basic field magnet 1 (for example, an axially superconducting air-coil magnet with active stray field shielding) that generates a uniform magnetic basic field in an interior space. The superconducting basic field magnet 1 is composed of superconducting coils in its interior that are contained in liquid helium. The basic field magnet 1 is surrounded by a two-shell vessel that is usually composed of stainless steel. The inner vessel that contains the liquid helium and also partly serves as winding member for the magnetic coils is suspended at the outer vessel, which is at room temperature, via Gfk rods with poor thermal conductivity. A vacuum exists between inner and outer vessel.
A cylindrical gradient coil arrangement 2 is concentrically introduced with carrier elements 7 into the interior of the basic field magnet 1 in the inside of a carrier tube. The carrier tube is limited toward the outside by an outer shell 8 and is limited toward the inside by an inner shell 9.
The gradient coil arrangement 2 has three partial windings that generate respective gradient fields that are proportional to the respective currents supplied thereto and that are spatially perpendicular to one another. As shown in FIG. 7, the gradient coil arrangement 2 has a x-coil 3, a y-coil 4 and a z-coil 5 that are respectively wound around the coil core 6 and thus generate respective gradient field, expediently in the direction of the Cartesian coordinates x, y and z. Each of these coils is equipped with its own power supply in order to generate independent current pulses in conformity with the sequences programmed in the pulse sequence controller that are exact both in terms of amplitude and time. The required currents lie at approximately 250 A. Since the gradient switching times should be as short as possible, current rise rates on the order of magnitude of 250 k A/s are necessary.
Since the gradient coil usually is surrounded by conductive structures (for example, magnet vessel of stainless steel), the pulsed fields create eddy currents in those structures that in turn interact with the basic magnetic field and vary it (i.e., they disturb its homogeneity).
In magnetic resonance tomography, however, the homogeneity of the basic magnetic field in the measurement volume is of basic significance.
Since the magnetic resonant frequency is directly dependent on the magnetic field strength, the same field strength should prevail in the defined measurement volume at every point in this volume. This is critical for the spatial resolution in imaging and for the reproducibility of frequency spectra in spectroscopic exams wherein field distortions caused by the susceptibility of the examination subject must be re-corrected.
Three different techniques are known for homogenization of the basic magnetic field (referred to below as shimming):
1. The three coils that are orthogonal to one another in the gradient coil 2 (FIG. 5, FIG. 7) for generating the gradient fields or for encoding the measurement volume are charged with offset currents in order to compensate field inhomogeneities of the first order.
2. A further, current-permeated orthogonal coil system with which it is likewise possible to homogenize the basic magnetic field is located within this gradient coil 2. these additional correction coils 10 (shim coils) (FIG. 5) thereby serves the purpose of compensating inhomogeneities of a higher order and therefore have an extremely complicated structure.
3. For homogenization of the basic magnetic field, a suitable arrangement of iron plates 11 referred to as shim iron plates (FIG. 5) is attached in the magnetic bore, i.e. within the gradient coil or between coil and basic field magnet. The locations where the plates 11 are attached are calculated with a field calculating program. A prior measurement of the field distribution serves as a prescription for the calculation. After assembly, a monitoring measurement is also implemented. This procedure must be multiply repeated before a satisfactory shim result has been achieved.
The first and second techniques represent an active shimming, the third technique is referred to as passive shimming. As a rule, a cooling water system is present for cooling the shim coils and shim iron plates.
Nonetheless, the overall condition for which shimming is needed is subject to different fluctuations due to changes in various physical parameters, thus inhomogeneities in the basic magnetic field still can arise both in terms of time and location.